Mirror Specifications and Energy-Efficient Design Support Increasingly Portable System for Laser Precision at Room Temperature

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Mirror setup 'squeezes' light, reducing quantum noise

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CAMBRIDGE, Mass., July 30, 2020 — A team of MIT physicists, working with researchers from Louisiana State University (LSU) and Crystalline Mirror Solutions (CMS, now part of Thorlabs Inc.) has designed a quantum “light squeezer” capable of limiting quantum noise in laser beams. The system retains the full range of quantum mechanical properties at room temperature. Researchers demonstrated the ability to apply the system in a comparatively compact configuration, meaning they may be able to add the system to high-precision experiments that improve laser measurements in settings where quantum noise may otherwise be a restricting factor.

The system’s name derives from the concept of quantum squeezing. This idea, that the uncertainty of a laser’s quantum properties stemming from the difficulty in exactly determining the number and timing of a laser’s photons, involves using a theoretical circle to represent laser photon momentum (number) and position (timing). Whereas a perfect circle symbolizes an equal degree of uncertainty in both properties, an ellipse or squeezed circle indicates an imbalance between the prominence of one property over the other. The way in which scientists manipulate and visualize the circle depends on the specific ratio of uncertainty in one or both quantum properties.

The lead author of the paper publishing the results is Nancy Aggarwal, formerly a graduate student in the MIT LIGO Laboratory and now a postdoctoral fellow at Northwestern University. Nergis Mavalvala, the Marble Professor of Astrophysics and associate head of physics at MIT, is one of nine additional co-authors.

Squeezed states of light can, in principle, improve a variety of optical measurements,” Mavalvala said. “They have already proven very useful in improving the sensitivity of currently operational interferometric gravitational-wave detectors. Furthermore, they can also play a role in improving the fault tolerance of quantum algorithms with quantum bits encoded on light, and for quantum key distribution.”

Mirror precision

The physical construct of the squeezer features a pair of precisely designed mirrors. The mirrors are placed in an optical cavity the size of a marble, which itself resides in a vacuum chamber. One mirror, the “nanomechanical” mirror, suspends from a cantilever (like a spring) and swings into motion when force from a laser’s light hits. The other mirror is stationary. When laser light bounces off the mirrors, their positioning allows the researchers to engineer the light leaving the cavity to possess special quantum properties.

This includes in a squeezed state. The research team used light in this condition to perform precise measurements, including for quantum computations and cryptology.

In diameter, the nanomechanical mirror is less than that of a human hair. Garrett Cole, of Thorlabs Inc, designed the mirrors for the experiments. Nearly 20 years ago, Cole began building micromechanically tunable lasers, incorporating small suspended and moveable mirrors targeting telecom applications. He later applied his expertise in these devices to trace-gas sensing and medical imaging in the sector of optical sensors.

In 2007, he shifted his focus to cavity optomechanics — a field that relies heavily on high-performance nanoscale mirrors. This work required Cole to significantly refine the mirrors that were sufficient for research and experiments involving the tunable lasers with which Cole was working at the time.

For these quantum regime experiments, [the mirrors] had to be exceptional as light as possible, as low-frequency as possible, as low of an elastic loss as possible (very high mechanical Q), and with the ultimate optical properties as low scatter and absorption loss as possible,” Cole said.

“The mirror in this case is a small disk, approximately 70 μm in diameter and roughly 4 μm thick,” Cole said. The cantilever suspending the mirror is made of single-crystal gallium arsenide that has a thickness of 220 nm, a width of 8 μm, and a length of 55 μm.”

“Overall, the mirror and supporting cantilever have a mass of 50 ng,” Cole said.

An MIT-designed miniature “squeezer” reduces quantum noise in lasers at room temperature. The marble-sized system could enable better laser precision for quantum computing and gravitational-wave detection. This illustration shows an artist’s interpretation of the system

An MIT-designed miniature 'squeezer' reduces quantum noise in lasers at room temperature. The marble-size system could enable better laser precision for quantum computing and gravitational-wave detection. This illustration shows an artist’s interpretation of the system. Courtesy of Christine Daniloff, MIT.

What the exactitude of the mirrors helps achieve in the system is optomechanical squeezing, a common method for carrying out quantum squeezing. Optomechanical squeezing depends on the ability of components such as mirrors to angle degree by degree to meet incoming laser light, due to a force that the photons making up the light impart on a mirror.

That force is proportional to the number of photons hitting the mirror at a given time. The distance the laser light force moves the mirror connects to the timing of the photons meeting the mirror.

Though the procedure does establish a useful correlation between position and momentum, cutting into the laser’s overall quantum noise to an extent, the results are imprecise. Adding to the shortcomings of the dynamic is the fact that a surrounding thermal energy is present in optomechanical squeezing. This causes a jitter that overwhelms contribution and influence from quantum noise.

Until now, scientists have relied on large setups in cryogenic freezers to house and realize optomechanical squeezing. “The minute you need cryogenic cooling, you can’t have a portable, compact squeezer,” Mavalvala said. “That can be a showstopper, because you can’t have a portable compact squeezer that lives in a big refrigerator, and then use it in an experiment or some device that operates it in the field.”

Cole and his team at CMS constructed the 70-μm mirrors for the system from alternating layers of single-crystal gallium arsenide and aluminum gallium arsenide. The materials combat the need for cryogenic cooling because they exhibit minimal thermomechanical losses at room temperature.

“Very disordered materials can easily lose energy because there are lots of places electrons can bang and collide and generate thermal motion,” Aggarwal said. Both gallium arsenide and aluminum gallium arsenide have detailed, ordered atomic structure, limiting space and potential for the loss of energy.

“For this specific effort, I worked with the groups at MIT and LSU to develop mirrors that met their requirements,” Cole said. “Target mass less than 100 ng, target (mechanical) resonant frequency less than 1 kHz, a target transmission of approximately 250 parts per million (ppm), and as high of a mechanical Q as possible at both room and cryogenic temperatures. To begin the device design, I defined the layer stack for the multilayer mirror and supporting cantilever.”

An outside vendor grew the crystal while Cole and his team, consisting of process engineers Paula Heu and David Follman (both included in the paper), designed a lithographic mask set and manufactured the devices at the University of California, Santa Barbara’s nanofabrication facility.

“Even just packing and shipping these parts was a challenge,” Cole said. “We had special sample holders designed and machined at MIT and LSU for transporting these extremely fragile structures.”

Industry applications

Once the system was complete, the team, led by LSU’s Thomas Corbitt, installed it in a laser experiment. Using the new squeezer and setup, the researchers effectively characterized the quantum fluctuations in the number of photons versus their timing, as the laser bounced and reflected off the mirrors. The team reduced the quantum noise from the laser by 15% while producing a more precise squeezed light.

“As optomechanical squeezers become more practical, this is the work that started it, Mavalvala said. “It shows that we know how to make these room-temperature, wavelength-agnostic squeezers. As we improve the experiment and materials, we will make better squeezers."

To make a truly pocket-size optomechanical squeezed light source at room temperature, we need micromirrors with low thermal noise and high optical quality. This will be achieved in the future with higher-purity crystalline materials that have lower mechanical dissipation, and with more robust and scalable fabrication techniques than we currently have. Technologies for low-loss transport of squeezed light in optical waveguides would also need to advance,” Mavalvala said.

The team continues to conduct experiments. Numerous industrial applications are driving advances in crystalline materials to improve their electronic, optical, and mechanical properties, Mavalvala said. “In a separate set of experiments, we are also testing and optimizing transport of squeezed states of light in optical fibers. These are some of the building blocks that will enable compact, portable, squeezed light sources,” she said.

Published: July 2020
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